The present invention relates to materials handling vehicles and, more particularly, to a motion control system that provides automatic engagement and disengagement of a coast control device of an electric pallet truck whose operator may either walk alongside or ride on the vehicle.
An industrial materials handling vehicle such as a lift truck typically includes a load bearing fork, a steerable wheel, a steering control mechanism, a brake that includes a deadman brake mechanism, an electric traction motor, and a storage battery. The steering mechanism commonly includes a handle mounted at the end of a movable arm and including rotatable twist grips that control the speed and direction of the truck in both forward and reverse directions as well as controls for raising and lowering the fork. The handle may also include switches for reversing vehicle travel direction and sounding a warning horn.
Motorized hand/rider trucks, which can be operated by an individual either walking alongside or standing on a platform incorporated in the vehicle structure, are widely used for various materials handling applications. One particular type of hand/rider truck, a low lift order picker truck, is employed for picking stock in large warehouses. In such an operation, the operator typically follows an established route through the warehouse bays, walking alongside the truck when the distance between items of stock to be picked is short and stepping on the truck platform to ride when the distance between picks is longer. When the operator is riding on the truck, it is desirable for optimum work productivity to move the truck at higher speeds than when the operator is walking beside it. Speed controllers that include high and low speed control circuits and switches are known in the art.
Electric trucks commonly include a deadman brake mechanism that is maintained in a disengaged position by the operator bringing the steering arm into an operating range. If the operator leaves the truck, a handle return spring forces the arm out of the operating range, which causes actuation of a spring-loaded brake to stop the vehicle, thus preventing injury to the operator and other workers as well as material damage. The lift truck may be in either a braking or non-braking mode, depending on the position of a steering arm within specified braking and driving arcs. The steering arm may be restrained from entering the braking arc by manual engagement of a deadman brake override device.
Considerations of safety have led to the design of electric vehicle operation control systems that automatically respond to the position of the operator, who may be standing or sitting on the vehicle, or walking alongside. A variety of plural operating modes for electric trucks are known in the art. For example, braking and other functions may be controlled by an operator either standing or seated on a vehicle, with interlock switches provided to ensure proper operation of the vehicle in accordance with industry standards.
As noted previously, the steering mechanism of a lift truck commonly includes rotatable twist grips. Rotation of the grips in one direction causes the truck to move forward; rotation of the grips in the opposite direction causes the truck to move in reverse. Increasing rotation of the grip in either direction results in an increase in the power supplied to the electric motor, causing the truck to move at higher speed in either a forward or reverse direction.
In addition to the motion control provided by the rotatable twist grips, motorized low lift order picker trucks may also include a side, or "jog", switch, which can be turned on and off by an operator walking alongside the truck, enabling the vehicle to continue to move from one stock pick up position toward the next.
The efficiency of stock picking operations would be severely hampered if the brake were activated every time the operator leaves the truck to make a pick up. Thus, brake override, or coasting, systems have been devised to allow the deadman brake mechanism to be disengaged while the operator walks alongside the truck. Low lift order picker trucks with coasting systems that override the deadman brake mechanism are required by industry safety standards to provide means for the walking operator to apply the service brake and sound the warning horn as needed. Furthermore, the maximum speed allowable with such coasting systems must not exceed normal walking speed, about 3.5 mph (5.6 km/hr). One such system for deactivating a deadman brake is described in U.S. Pat. No. 4,716,980 which issued on Jan. 5, 1988 and is entitled "Control System for Rider Vehicles". In that system, a selector switch is used to select either normal high speed operation or coasting operation and additional switches which are independent of the selector switch are then used to drive a truck within one of two speed ranges depending on the operating mode selected.
Coasting systems for order picker trucks like the system described in the above referenced patent, while providing some facilitation of stock picking operations, require manual engagement of the deadman brake override control device by the vehicle operator to put the truck in its coast mode or disengage the coast mode for high speed travel. While infrequent manual activation of the deadman brake override control device might not be burdensome, in practice manual activation is a regular operation and repeated manual activation can be extremely tedious.
For at least this reason, a device that utilizes electrical signals from the truck's motion control circuits to provide appropriate automatic override of the deadman brake mechanism, thereby enhancing stock picking efficiency while maintaining operational safety requirements, would be highly advantageous. The motion control system and coast control device of the present invention provide this benefit.